Membrane electrode assembly having protective layer and method for mitigating membrane decay

ABSTRACT

A membrane electrode assembly includes an anode; a cathode; a membrane disposed between the anode and the cathode; and a protective layer positioned between the membrane and the cathode, the protective layer having a first side and a second side and being adapted to restrict migration of oxygen at the first side and to restrict the migration of hydrogen at the second side and thereby maintain a plane of potential change between the anode and the cathode within the protective layer.

BACKGROUND OF THE DISCLOSURE

The disclosure relates to fuel cells and, more particularly, to PEM fuelcells and reduction in degradation of the membrane of same.

In a PEM fuel cell, various mechanisms can cause peroxide to form orexist in the vicinity of the membrane. This peroxide can dissociate intohighly reactive free radicals. These free radicals can rapidly degradethe membrane.

It is desired to achieve 40,000-70,000 hour and 5,000-10,000 hourlifetimes for stationary and transportation PEM fuel cells,respectively. Free radical degradation of the ionomer seriouslyinterferes with efforts to reach these goals.

It is therefore the primary object of the present disclosure to providea membrane electrode assembly which addresses these issues.

It is a further object of the disclosure to provide a method foroperating a fuel cell which further addresses these issues.

Other objects and advantages appear herein.

SUMMARY OF THE DISCLOSURE

In accordance with the present disclosure, the foregoing objects andadvantages have been attained.

According to the disclosure, a membrane electrode assembly is providedwhich comprises an anode; a cathode; a membrane between the anode andthe cathode; and a protective layer between the membrane and thecathode, the protective layer being adapted to block oxygen at one sideand hydrogen at the other side and thereby maintain a plane of potentialchange between the anode and the cathode within the protective layer.

In further accordance with the disclosure, a method is provided formitigating decay of a membrane electrode assembly, which methodcomprises selectively operating a membrane electrode assembly in anon-load condition and an off-load condition, the membrane electrodeassembly having an anode, a cathode, a membrane between the anode andthe cathode, and a protective layer between the membrane and thecathode, wherein a plane of potential change between the anode and thecathode falls within the protective layer in both the on-load conditionand the off-load condition.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of preferred embodiments of the presentdisclosure follows, with reference to the attached drawings, wherein:

FIG. 1 schematically illustrates a membrane electrode assembly includinga protective layer in accordance with the present disclosure; and

FIG. 2 illustrates potential through a portion of the assembly due tothe protective layer of the present disclosure.

DETAILED DESCRIPTION

The disclosure relates to fuel cells and, more particularly, to polymerelectrolyte membrane (PEM) fuel cells, and to mitigating decay ordegradation of such fuel cells due to on-load and off-load operation andcycling between same.

FIG. 1 schematically illustrates a membrane electrode assembly (MEA) 10in accordance with the disclosure. As shown, assembly 10 includes amembrane 12, a cathode 14, an anode 16, and gas diffusion layers 18, 20.According to the disclosure, a protective layer 22 is also provided,preferably between membrane 12 and cathode 14. Cathode 14 and anode 16are positioned to either side of membrane 12 as shown, with gasdiffusion layers 18, 20 positioned to either side of the electrodes(cathode 14 and anode 16).

As is well known to a person skilled in the art, membrane electrodeassembly 10 is operated by feeding oxygen in some form through gasdiffusion layer 18 to cathode 14 and by feeding hydrogen in some formthrough gas diffusion layer 20 to anode 16. These reactants supportgeneration of an ionic current across membrane 12 as desired.

Cathode 14 is a porous layer containing a suitable cathode catalyst andtypically having a porosity of at least about 30%. Anode 16 is similarlya porous layer containing suitable anode catalyst, and also typicallyhas a porosity of at least about 30%.

During operation of assembly 10, catalyst materials which are typicallypresent within the electrodes, that is, cathode 14 and/or anode 16, candissolve and then precipitate elsewhere in the assembly.

It has been found, as shown in FIG. 2, that during operation of the MEA,there is a plane of sharp potential change between the electrodes, andthis plane of potential change is referred to as Xo 23. At Xo 23,reaction potential abruptly shifts from a low value to a high value. Theposition of Xo 23 depends heavily on the oxidant and reductant gasconcentrations at locations on either side of Xo 23. If electricallyisolated catalyst particles are present at Xo 23, this is a very likelyposition for formation of peroxide and/or generation of radicals whichcan have a deleterious effect upon membrane 12 and other ionomer presentwithin assembly 10.

It has further been found that dissolved catalyst metal tends toprecipitate or deposit at Xo 23, and that this deposited metal canincrease the chance of formation of peroxide and/or radicals. Peroxidehas been found to be directly responsible for degradation of membrane12, because peroxide under certain conditions can break down to formradicals which react with the membrane and then carry portions of themembrane out of assembly 10 through exhaust from same. Also, radicals(e.g. hydroxyl or peroxyl) may form directly on such catalystprecipitates from the reaction of crossover gases and/or peroxide, whichproceed to degrade the membrane.

During electrical load cycling of assembly 10, the concentration ofreactants varies and the position of Xo 23 can move. When this happens,there is increased tendency toward dissolution of catalyst metal fromthe previous Xo 23 location to the new Xo 23 location. This is becauseafter a certain amount of time of operation, sufficient metal depositsat Xo 23 that there is less driving force for dissolution. However, whenXo 23 moves, additional dissolution of catalyst can take place from boththe electrodes and from catalyst particles already deposited in themembrane. Such a process can be especially damaging to the membrane dueto the high specific area of catalyst surface that results. As set forthabove, in accordance with the disclosure, protective layer 22 isutilized to keep Xo 23, the plane of potential change, at a particularposition during normal, or on-load operation, and further steps aretaken during off-load operation to further maintain Xo 23 in a desiredposition.

According to the disclosure, several embodiments of protective layer 22are provided, each of which serves to restrict migration of hydrogen andoxygen. Oxygen is restricted at a side of layer 22 which faces thecathode, and hydrogen is restricted at the other side of layer 22. Thisresults in Xo 23 remaining within layer 22 as desired.

The disclosed embodiments include catalyzed layers which chemicallyscavenge oxygen and hydrogen, for example forming water.

In one embodiment, protective layer 22 is advantageously a layer ofionomer material preferably containing a catalyst, the catalystpreferably in particulate form. The protective layer 22 preferably has aporosity of less than about 10% by volume (most preferably non-porous),contains between about 50% and about 80% vol ionomer, and between about10% and about 50% vol catalyst. Electrical connectivity between thecatalyst particles is preferably between about 35% and about 95%. Thesecatalyst particles in the protective layer 22 are substantiallyelectrically connected to the cathode. They may alternatively or evenpreferably be electrically connected by a high surface area supportmaterial.

During normal operation of assembly 10, protective layer 22 issubstantially electrically connected to cathode 14 and serves toscavenge any oxygen which would otherwise cross over into membrane 12and also scavenges hydrogen, which has crossed through membrane 12.Because of this, Xo 23 is forced to reside within protective layer 22during on-load operation. Protective layer 22 may also serve todecompose any peroxide formed, for example at cathode 14.

During off-load operation, no scavenging of crossover oxygen takes placein protective layer 22 since no current flows in the cell. In thissituation, an air starvation protocol is implemented, whereby the oxygennormally fed to gas diffusion layer 18 to cathode 14 is instead stoppedor redirected away from cathode 14, for example by being vented toambient conditions instead. While protective layer 22 under off-loadconditions has reduced effectiveness at scavenging oxygen, the air oroxygen starvation protocol provides the same effect, which tends to keepXo 23 within protective layer 22 during off-load conditions as well.Such a protocol also limits the high potential that the cathode wouldotherwise experience by allowing crossover hydrogen to reduce thecathode potential.

Different types of ionomer and catalyst material can be used in thisembodiment. As will be further discussed, the protective layer in thisembodiment serves to scavenge crossover gasses by having a high gasreaction rate and a low gas diffusion rate. The protective layer furtherserves to maximize selectivity to benign products, preferably water,from such crossover gasses and may serve to decompose peroxide. Inaddition, since the protective layer is intended, according to thedisclosure, to contain Xo 23, the protective layer structureadvantageously discourages the loss of catalyst from the electrodes asdiscussed below.

The catalyst in protective layer 22 is preferably largely electricallyconnected, and protective layer 22 therefore serves as a sink fordeposition of dissolved catalyst metal, and the dissolution drivingforce is reduced or eliminated. Thus, keeping Xo 23 within protectivelayer 22 minimizes or eliminates the driving force under both on andoff-load operating conditions.

In accordance with this embodiment, the protective layer 22 comprises acatalyst, for example, carbon supported platinum or platinum alloyparticles, the pores of which are filled with polymer electrolyte, orionomer material. When platinum alloys are used, the catalyst particlescan advantageously be binary and/or ternary alloys, and can besupported, for example on carbon, or non-supported.

One suitable platinum alloy has the formula Pt_(x)Y_(1−x), wherein Y isselected from the group consisting of Co, Ni, Ir, Rh, V, Cu, Fe, Cr, Pd,Ti, W, Al, Ag, Cu and combinations thereof, and x is between 0.1 and0.9.

According to a further embodiment of the disclosure, the platinum alloycan have the formula Pt_(x)M_(z)Y_(1−x−z), wherein: M is selected fromthe group consisting of Ir, Rh, Co, Ni and combinations thereof; Y isselected from the group consisting of Co, Ni, V, Cu, Fe, Cr, Pd, Ti, W,Al, Ag, Cu, Au and combinations thereof; and x+z is between 0.1 and 0.9.

According to a still further embodiment, the platinum alloy has theformula Pt_(x)Z_(1−x), wherein Z is selected from the group consistingof Ru, Mo, and combinations thereof, and wherein x is between 0.1 and0.9.

Other suitable catalysts, including other metal alloy catalysts, can beutilized. Alternatives may be apparent to a person of skill in the art.While the foregoing embodiments represent preferred configurations, suchalternatives are considered to be well within the broad scope of thepresent disclosure.

At the relatively high potential which will be present in protectivelayer 22, the four electron reduction of oxygen is predominatelyachieved so as to produce water and not produce peroxide. Thus, oxygenis scavenged by protective layer 22 at high potential as desired.

Protective layer 22 serves to consume such oxygen at high potential,most actively at the interface 21 between protective layer 22 andcathode 14. Protective layer 22 further serves to consume hydrogen atthe interface 24 between membrane 12 and protective layer 22. Further,protective layer 22 also provides for benign decomposition of peroxideat interface 24 and throughout the thickness of the layer 22 if peroxideis generated in cathode 14 and/or at interface 24 and throughout thethickness of layer 22 if peroxide is generated in anode 16. Thesefunctions advantageously reduce a significant contributor toward celldegradation.

In order to provide desirable results, protective layer 22 isadvantageously electrically connected to cathode 14 through anelectrically conducting phase, for example such as carbon supportmaterial, so as to ensure high potential and, therefore, consumption ofcrossover oxygen to produce water. Unsupported catalyst particles mayalso be electrically connected to the cathode through an interconnectednetwork.

Protective layer 22 further preferably has substantially no porosity anda relatively high oxygen reduction rate which is preferablysubstantially the same as, or greater than, the oxygen reduction rate ofthe cathode. This will result in a maximized ratio of oxygen reductionrate to oxygen diffusion rate, and thereby will minimize oxygen escapefrom the cathode.

In this regard, protective layer 22 advantageously has a porosity ofless than about 10%, and is preferably substantially non-porous(substantially 0% porosity). Oxygen reduction rate per unit platinumsurface area for protective layer 22 is also advantageouslyapproximately the same as the cathode because of electrical connectivityto the cathode.

Any porosity of protective layer 22 should advantageously be floodedduring operation, for example with water, so as to reduce the oxygendiffusion rate through the protective layer 22. A layer 22 havingporosity which is flooded with water during normal operation isconsidered to be non-porous as used herein since the water-filledporosity is substantially less porous to reactant gasses.

Provision of a protective layer 22 having these propertiesadvantageously results in efficient oxygen consumption at interface 21and throughout layer 22 and, therefore, proper conditions for keeping Xo23 within layer 22 during the on-load operating conditions.

As set forth above, providing protective layer 22 between cathode 14 andmembrane 12 advantageously serves to define Xo 23 within protectivelayer 22 as desired, thereby allowing for reduced chance of catalystdriven generation of peroxide and catalyst driven formation of radicals,and also minimizing movement of Xo 23 such that a sink of catalystmaterial can be initially provided in protective layer, or initiallydeposited in protective layer 22 during early operation, to therebyreduce or eliminate the driving force for catalyst dissolution duringon-load operation.

Protective layer 22 can be provided using various ionomer materials asdiscussed above, and advantageously serves to force Xo 23 to stay withinprotective layer 22 as desired.

In further accordance with the disclosure, protective layer 22 caninclude a hydrocarbon (non-fluorinated) ionomer, or a per-fluorinatedionomer (such as Nafion), or a combination, for example, bysubstantially homogeneously blending hydrocarbon in liquid ionomer orparticulate form into the per-fluorinated ionomer-based material.

During off-load operation, oxidant is re-directed away from cathode 14,and this serves to maintain oxygen depletion in the vicinity ofprotective layer 22 and thereby to keep Xo 23 within protective layer 22as desired.

It should be appreciated that the present disclosure, drawn to reductionof degradation due to on-load and off-load operation, and cyclingbetween them, advantageously accomplishes this purpose. Furthermore, itshould be appreciated that the disclosure is useful in connection with awide variety of different types of ionomer in the protective layer, allwithin the broad scope of the present disclosure.

While the present disclosure has been described in the context ofspecific embodiments thereof, other alternatives, modifications, andvariations will become apparent to those skilled in the art having readthe foregoing description. Accordingly, it is intended to embrace thosealternatives, modifications, and variations as fall within the broadscope of the appended claims.

1. A membrane electrode assembly, comprising: an anode; a cathode; amembrane between the anode and the cathode; and a protective layerbetween the membrane and the cathode, the protective layer having firstand second opposed sides and being adapted to restrict oxygen at thefirst side and to restrict hydrogen at the second side and therebymaintain a plane of potential change between the anode and the cathodewithin the protective layer.
 2. The assembly of claim 1, wherein theprotective layer comprises a layer of ionomer material containingcatalyst selected to scavenge at least one of hydrogen and oxygen. 3.The assembly of claim 1, wherein the protective layer has an oxygenreduction rate which is substantially the same or greater than thecathode.
 4. The assembly of claim 2, wherein the protective layercomprises catalyst particles selected from the group consisting ofparticles of carbon, particles of platinum, particles of platinum alloyand combinations thereof.
 5. The assembly of claim 4, wherein thecatalyst particles comprise particles of platinum or particles ofplatinum alloy, and wherein the catalyst particles are supported oncarbon.
 6. The assembly of claim 4, wherein the particles compriseplatinum alloy selected from the group consisting of binary alloys,ternary alloys and combinations thereof.
 7. The assembly of claim 6,wherein the platinum alloy has the formula Pt_(x)Y_(1−x), wherein Y isselected from the group consisting of Co, Ni, Ir, Rh, V, Cu, Fe, Cr, Pd,Ti, W, Al, Ag, Cu and combinations thereof, and x is between 0.1 and0.9.
 8. The assembly of claim 6, wherein the platinum alloy has theformula Pt_(x)M_(z)Y_(1−x−z), and wherein: M is selected from the groupconsisting of Ir, Rh, Co, Ni and combinations thereof; Y is selectedfrom the group consisting of Co, Ni, V, Cu, Fe, Cr, Pd, Ti, W, Al, Ag,Cu, Au and combinations thereof; and x+z is between 0.1 and 0.9.
 9. Theassembly of claim 6, wherein the platinum alloy has the formulaPt_(x)Z_(1−x), wherein Z is selected from the group consisting of Ru,Mo, and combinations thereof, and wherein x is between 0.1 and 0.9. 10.The assembly of claim 2, wherein the protective layer is electricallyconnected to the cathode.
 11. The assembly of claim 4, wherein thecatalyst particles in the protective layer are substantiallyelectrically connected to the cathode.
 12. The assembly of claim 11,wherein the catalyst particles are electrically connected to the cathodevia a high surface area support material.
 13. The assembly of claim 1,wherein the protective layer has a porosity of less than 10%.
 14. Theassembly of claim 13, wherein the protective layer is substantiallynon-porous.
 15. The assembly of claim 1, wherein the protective layer isan electrically connected and ionically conductive structure having aporosity of between 0% and 10%, wherein the catalyst is present in anamount between 10% and 50% vol based upon volume of the layer, andion-exchange material is present in an amount between 50% and 80% volbased upon volume of the layer.
 16. A method for mitigating decay of amembrane electrode assembly, comprising selectively operating a membraneelectrode assembly in an on-load condition and an off-load condition,the membrane electrode assembly having an anode, a cathode, a membranebetween the anode and the cathode, and a protective layer between themembrane and the cathode, wherein a plane of potential change betweenthe anode and the cathode falls within the protective layer in both theon-load condition and the off-load condition.
 17. The method of claim16, wherein operation in the off-load condition comprises stopping flowof oxidant to the cathode or re-directing the oxidant away from thecathode.
 18. The method of claim 16, wherein the protective layercomprises a layer of ionomer material containing catalyst selected toscavenge at least one of hydrogen and oxygen.
 19. The method of claim16, wherein the protective layer has an oxygen reduction rate which issubstantially the same or greater than the cathode.
 20. The method ofclaim 16, wherein the protective layer comprises catalyst particlesselected from the group consisting of particles of carbon, particles ofplatinum, particles of platinum alloy and combinations thereof.
 21. Themethod of claim 20, wherein the catalyst particles comprise particles ofplatinum or particles of platinum alloy, and wherein the catalystparticles are supported on carbon.
 22. The method of claim 20, whereinthe particles comprise platinum alloy selected from the group consistingof binary alloys, ternary alloys and combinations thereof.
 23. Themethod of claim 22, wherein the platinum alloy has the formulaPt_(x)Y_(1−x), wherein Y is selected from the group consisting of Co,Ni, Ir, Rh, V, Cu, Fe, Cr, Pd, Ti, W, Al, Ag, Cu and combinationsthereof, and x is between 0.1 and 0.9.
 24. The method of claim 22,wherein the platinum alloy has the formula Pt_(x)M_(z)Y_(1−x−z), andwherein: M is selected from the group consisting of Ir, Rh, Co, Ni andcombinations thereof; Y is selected from the group consisting of Co, Ni,V, Cu, Fe, Cr, Pd, Ti, W, Al, Ag, Cu, Au and combinations thereof; andx+z is between 0.1 and 0.9.
 25. The method of claim 22, wherein theplatinum alloy has the formula Pt_(x)Z_(1−x), wherein Z is selected fromthe group consisting of Ru, Mo, and combinations thereof, and wherein xis between 0.1 and 0.9.
 26. The method of claim 16, wherein theprotective layer is electrically connected to the cathode.
 27. Themethod of claim 26, wherein catalyst particles in the protective layerare substantially electrically connected to the cathode.
 28. The methodof claim 27, wherein the catalyst particles and the cathode areconnected via a high surface area support material.
 29. The method ofclaim 16, wherein the protective layer has a porosity of less than 10%.30. The method of claim 29, wherein the protective layer issubstantially non-porous.
 31. The method of claim 16, wherein theprotective layer is an electrically connected and ionically conductivestructure having a porosity of between 0% and 10%, wherein the catalystis present in an amount between 10% and 50% vol based upon volume of thelayer, and ion-exchange material is present in an amount between 50% and80% vol based upon volume of the layer.